This invention relates to a method of inhibiting scale formation and/or the formation of scale forming salts in boilers using a polymer admixture comprising polymers of different molecular weights. These synergistic polymer compositions are hydrothermally stable and provide improved threshold scale inhibition at elevated temperatures and pressures.
Certain types of water soluble polymers have been used in the past to fluidize precipitates, salts, or other solids which may be found in high temperature boiler water systems. For example, U.S. Pat. No. 3,630,937, entitled "Boiler Water Treatment", discloses the use of water soluble sulfonated polystyrenes, alone or in combination with other boiler chemicals. U.S. Pat. No. 2,783,200 discloses the use of certain polyacrylate compounds, and U.S. Pat. No. 3,514,376 discloses the use of short chain polyacrylic acid (molecular weight:5,000-13,000), to prevent scale formation in boilers and evaporators, respectively. U.S. Pat. No. 2,980,610 discloses the use of polyacrylamides for inhibiting the precipitation of calcium and other salts in boiler waters. Additionally, U.S. Pat. Nos. 3,709,815 and 3,928,196 disclose the use of polymers containing sulfonic acid and carboxylic acid moieties as scale inhibitors for aqueous systems.
Most industrial waters contain alkaline earth metal cations, such as calcium, barium, magnesium, etc. and several anions such as bicarbonate, carbonate, sulfate, oxylate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until these product solubility concentrations are no longer exceeded. For example, when the concentrations of calcium ion and carbonate ion exceed the solubility of calcium carbonate reaction products, a solid phase of calcium carbonate forms.
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, change of pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in solution. As these reaction products precipitate on surfaces of a water carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. This scale is an expensive problem in many industrial applications, causing delays and shut downs for cleaning and removal.
The inventor has discovered that polymer blends comprising polymers of different molecular weights are synergistic with respect to both hydrothermal stability and threshold scale inhibition at elevated temperatures and pressures. The use of these novel polymer blends to treat boiler systems is unknown in the art.